Gold-standard cyanide detection: IC and a gold electrode can detect cyanide in water

Ezine

Published: Sep 11, 2017

Author: Jon Evans

Channels: Ion Chromatography

Safe thresholds

One chemical compound that you certainly don’t want to find in your drinking water is cyanide. This combination of a carbon atom and nitrogen atom connected via a triple bond is highly toxic, interfering with cellular respiration in the body and quickly leading to seizures, heart attacks and death.

Unfortunately, drinking water can occasionally contain cyanide, derived both from natural sources such as certain microbes and from industrial activities such as gold mining. As long as the concentrations are low enough, however, cyanide doesn’t present any great risk to humans or wildlife, with regulatory authorities tending to set thresholds of 50–200μg/L.

Ion chromatography with amperometric detection has proved to be an effective method for checking that these thresholds aren’t being exceeded. This is because it doesn’t suffer as much interference from other compounds in the water, such as ions and metals, as many alternative methods for determining cyanide concentrations.

But it does still struggle with sulfides and chlorides, which like cyanide generate an electric current when oxidized or reduced at a special electrode, usually made from silver. This is a particular problem for chlorides, which are reduced to silver chloride that ends up coating the electrode and hampering the detection of cyanide.

Clean electrode

To overcome this problem, Amit Bansiwal and his colleagues at the National Environmental Engineering Research Institute in Nagpur, India, decided to experiment with replacing the conventional silver electrode with a gold one, which had never been tried before. Using a solution of cyanide, they first determined how they should vary the voltage applied to the gold electrode over time, known as the waveform, to produce the largest current when the cyanide is reduced.

They also designed the waveform so that it regularly cleans the electrode. The reduction of cyanide at a gold electrode relies on the associated oxidation of gold to gold oxide, which eventually covers the electrode, hampering its detection of further cyanide. To prevent this happening, Bansimal and his team simply varied the voltage such that the gold oxide was periodically reduced back to gold.

Next, they coupled their gold electrode with ion chromatography and tested it on water spiked with various concentrations of cyanide. This revealed that their method could reliably determine cyanide in water at concentrations of between 5μg/L and 8000μg/L, and had a limit of detection of just 2μg/L. This makes it just as good as the most sensitive of the existing techniques and more than sensitive enough to assess whether water meets current regulatory thresholds.

Interfering ions and metals

To test whether this sensitivity might be adversely affected by other compounds in the water, they also analyzed water spiked with cyanide and either ions like chloride, sulfide and phosphate or metals like iron, copper and zinc. Many of the ions, including chloride, simply weren’t detected by the gold electrode, while sulfide was detected but didn’t interfere with the detection of cyanide. Other studies have suggested removing all metals from water before trying to detect cyanide, because of the high level of interference. In contrast, Bansimal and his team found their method didn’t suffer from any interference at all.

Finally, they used the method to check for cyanide in samples of tap water, bottled water and groundwater, but didn’t find any. When they spiked these samples with cyanide at concentrations of 2μg/L, 20μg/L or 50μg/L, however, they spotted it straight away, confirming that none of the other compounds in the water samples were causing any interference.

An added advantage of this combination of ion chromatography and gold-based amperometric detection is that, unlike some other methods, it doesn’t generate harmful hydrogen cyanide gas during analysis. Because you also don’t want cyanide floating around your laboratory.